1. Field of the Invention
The invention relates to a cylinder-to-cylinder air/fuel ratio (A/F) imbalance determination system provided in a multi-cylinder internal combustion engine, for determining (monitoring, detecting) whether a significant imbalance occurs between the air/fuel ratios of air-fuel mixtures supplied to respective cylinders (the air/fuel ratios of respective cylinders) (i.e., whether a cylinder-to-cylinder air/fuel ratio (A/F) imbalance condition occurs).
2. Description of the Related Art
An air/fuel ratio control system is widely known which includes a three-way catalyst mounted in an exhaust passage of an internal combustion engine, and upstream air/fuel ratio sensor and downstream air/fuel ratio sensor disposed upstream and downstream, respectively, of the three-way catalyst in the exhaust passage. The air/fuel ratio control system calculates an air/fuel ratio feedback amount based on an output signal of the upstream air/fuel ratio sensor and an output signal of the downstream air/fuel ratio sensor, so that the air/fuel ratio of an air-fuel mixture supplied to the engine (the air/fuel ratio of the engine) becomes equal to the stoichiometric air/fuel ratio, and controls the air/fuel ratio of the engine in a feedback fashion, using the air/fuel ratio feedback amount. Another type of air/fuel ratio control, system has also been proposed which calculates an air/fuel ratio feedback amount based on only one of the output signal of the upstream air/fuel ratio sensor and the output signal of the downstream air/fuel ratio sensor, and controls the air/fuel ratio of the engine in a feedback fashion, using the air/fuel ratio feedback amount. The air/fuel ratio feedback amount used in the air/fuel ratio control system as described above is a control amount common to all of the cylinders of the engine.
Generally, an electronic fuel injection type internal combustion engine includes at least one fuel injection valve mounted in each cylinder or an intake port that communicates with each cylinder. Accordingly, if the fuel injection valve of a particular cylinder becomes characterized in that “the fuel injection valve injects an excessively large amount of fuel compared to an instructed fuel injection amount”, only the air/fuel ratio of an air-fuel mixture supplied to the particular cylinder (the air/fuel ratio of the particular cylinder) shifts largely to the rich side. Namely, the unevenness of the air/fuel ratio among the cylinders (or variation in the air/fuel ratio among the cylinders, or air/fuel ratio (A/F) imbalance among the cylinders) increases. In other words, an air-fuel ratio (A/F) imbalance occurs between one cylinder (particular cylinder) and the other or remaining cylinders.
In this case, the average air/fuel ratio of the air-fuel mixture supplied to the engine as a whole becomes richer than the stoichiometric air/fuel ratio. Accordingly, if the air/fuel ratios of the cylinders are feedback-controlled based on the air/fuel ratio feedback amount common to all of the cylinders, the air/fuel ratio of the particular cylinder is changed to the lean side so as to be closer to the stoichiometric air/fuel ratio, and, at the same time, the air/fuel ratio of the remaining cylinders is changed to the lean side, away from the stoichiometric air/fuel ratio. As a result, the average air/fuel ratio of all of the air-fuel mixtures supplied to the engine is made substantially equal to the stoichiometric air/fuel ratio.
However, as a result of the feedback control, the air/fuel ratio of the particular cylinder is still richer than the stoichiometric air/fuel ratio, and the air/fuel ratio of the remaining cylinders becomes leaner than the stoichiometric ratio; therefore, a combustion condition of the air-fuel mixture in each cylinder differs from that of complete combustion. As a result, an increased amount of emissions (i.e., increased amounts of unburned substances and nitrogen oxides) are discharged from each cylinder. Therefore, even if the average air/fuel ratio of the air-fuel mixtures supplied to the engine is substantially equal to the stoichiometric air/fuel ratio, the three-way catalyst may not fully or completely clean the thus increased emissions, resulting in deterioration of the emissions.
Accordingly, it is important to detect the excessive unevenness or imbalance of the air/fuel ratio among the cylinders (i.e., occurrence of a cylinder-to-cylinder A/F imbalance condition), and take some measures against this situation in an attempt to avoid deterioration of the emissions. In this connection, a cylinder-to-cylinder A/F imbalance occurs due to various factors, for example, when the fuel injection valve of a particular cylinder becomes characterized by injecting an excessively small amount of fuel compared to the instructed fuel injection amount, or when EGR gas or evaporative fuel gas is unevenly distributed into the respective cylinders.
A known system that determines whether a cylinder-to-cylinder A/F imbalance condition as described above occurs is configured to obtain a trace length of the output (output signal) of an air/fuel ratio sensor (the above-mentioned upstream air/fuel ratio sensor) disposed in an exhaust collecting portion into which exhaust gases are collected from a plurality of cylinders, compares the trace length with “a reference value that varies in accordance with the engine speed and the intake air amount”, and determines whether a cylinder-to-cylinder A/F imbalance condition occurs, based on the result of the comparison (see, for example, U.S. Pat. No. 7,152,594). In the following description, a determination as to whether “an excessive cylinder-to-cylinder A/F imbalance condition” occurs will be simply called “a cylinder-to-cylinder A/F imbalance determination, or an imbalance determination”. The “excessive cylinder-to-cylinder A/F imbalance condition” means a cylinder-to-cylinder A/F imbalance condition in which the amount of unburned substances and/or nitrogen oxides exceeds a prescribed value.
To accurately make an A/F imbalance determination using the output of the air/fuel ratio sensor, the output characteristics of the air/fuel ratio sensor are required to match well with “the output characteristics of a standard air/fuel ratio sensor”. The output characteristics of the standard air/fuel ratio sensor are those of “an air/fuel ratio sensor used when determining, in advance, a threshold value or values (e.g., the reference value used in the known system) based on which an A/F imbalance determination is made”. The output characteristics of the air/fuel ratio sensor are, for example, the output gain, response, and other characteristics of the air/fuel ratio sensor. The output gain is, for example, an amount proportional to the magnitude of a difference between an output value of the air/fuel ratio sensor obtained when the air/fuel ratio is equal to the stoichiometric air/fuel ratio, and an output value of the air/fuel ratio sensor obtained when the air/fuel ratio is equal to a given air/fuel ratio. The standard air/fuel ratio sensor will also be called “reference air/fuel ratio sensor”.
It is, however, to be noted that air/fuel ratio sensors have individual differences due to, for example, manufacturing variations. Also, the air/fuel ratio sensor may deteriorate with use, and the output characteristics of the sensor may change with time. Thus, if the output characteristics of the air/fuel ratio sensor noticeably differ from “the output characteristics of the reference air/fuel ratio sensor”, a parameter or parameters for use in cylinder-to-cylinder A/F imbalance determination, which are obtained based on the output value of the air/fuel ratio sensor, cannot be accurately obtained, and therefore, a cylinder-to-cylinder A/F imbalance determination cannot be made with high accuracy.